This invention relates generally to liquid scintillation counting methods and, more particularly, to improvements in methods for measuring the disintegration rate of a beta-emitting radionuclide in a liquid sample.
Liquid scintillation techniques have been widely adopted to measure the count rate or activity of samples containing radionuclides. The radioactive sample, typically a beta emitter, is placed in direct contact with a liquid scintillation medium by dissolving or suspending the sample within the medium. The liquid scintillation medium comprises a solvent or solvents, typically toluene or dioxane, and solute or solutes present in a few percent by weight of the solution. A liquid scintillation solution consisting of the solvent(s), a solute(s), and a radioactive sample are placed within a sample vial for measuring the radioactive emissions within the liquid scintillator. It is theorized that most of the kinetic energy from the nuclear disintegrations of the radioactive sample is absorbed by the solvent and then transferred to the solute which emits photons as visible light flashes or scintillations. The amount of emitted light is proportional to the amount of energy absorbed from the disintegrations. The scintillations are detected by a photomultiplier tube or other light responsive device which converts the energy of each scintillation to a voltage pulse having a pulse height proportional to the energy of the detected scintillation.
To derive a pulse height spectrum of a sample, the output electrical pulses from the photomultiplier are amplified and counted in a plurality of parallel counting channels. Each channel typically includes a pulse height analyzer with discriminators which establish a channel counting "window" having upper and lower pulse height limits. Each counting channel therefore counts the total number of pulses produced having pulse heights within the window limits of the channel. By establishing a plurality of counting channels having window settings which span a range of pulse heights and by counting the number of pulses falling within each channel, a pulse height spectrum is obtained for the particular radioactive sample. Since the output pulse heights from the photomultiplier are proportional to the energy of the corresponding scintillations, the pulse height spectrum corresponds to the energy spectrum of the nuclear radiation emitted by the test sample.
It is well known that radioactive samples or materials present in the scintillation medium can adversely affect the process by which the scintillations are produced. For example the emission of photons can be prevented or emitted photons can be absorbed. Further, some events can be reduced to a level which is below the minimum detection threshold of the photomultiplier. Such effects are commonly referred to as "quenching" and in each case result in a reduction in the number of photons detected by the photomultiplier. When quenching results in a reduction of the level of some events below the detection level of the photomultiplier, the measured count rate will be lower than that produced by the same amount of the radionuclide in an unquenched sample. This is commonly referred to as a decrease in "counting efficiency".
Quenching acts equally on all events produced by the same type of excitation particle, e.g. electron (beta), alpha, proton, etc. Thus if quenching is sufficient to reduce the measured response for one disintegration by a given percentage, it will reduce all given responses by the same percentage. The result is to shift the energy or pulse height spectrum to lower pulse height values, and this is commonly referred to as "pulse height shift".
A major effort has been directed to develop techniques for monitoring the level of quench and for correcting the measured pulse height response to compensate for the effect of quench. Typically, calibration curves which plot counting efficiency vs. the degree of quench are established, where counting efficiency is defined as the observed sample counts divided by the actual disintegrations within the sample. After an unknown sample has been counted and its degree of quench determined, the disintegrations within the sample are calculated from the previously established calibration curves.
In order to generate a standard counting efficiency vs. quench calibration curve, a known set of calibration standards must be counted. This is the accepted practice in present day, highly sophisticated liquid scintillation counters which are designed to count many samples on a high volume basis. When a large number of samples are to be counted, the relative time to preset the instrument and run the calibration standards may be relatively small. However, where a user is only interested in counting one or just a few samples, the time required to preset and calibrate the system may be unacceptably long.
A "double extrapolation" procedure for determining the disintegration rate of a radionuclide in a sample without using a standard solution of the radionuclide to calibrate the counting system is described in Nature, London 202 78 (1964), in Progr. Nucl. Energy, Ser. 9, 7 21-110 (1966), and in Organic Scintillators and Liquid Scintillation Counting, p. 687, Academic Press, New York, 1971. In the described procedure, a sample containing a beta-emitting radionuclide is successively quenched either by the addition of quenching agents to the sample solution or by introducing optical filters between the sample and the single phototube included in the counting system. In both cases, the sample and the quenched versions thereof are irradiated with a gamma source to produce Compton scattered electron pulse spectra for the samples in the presence of the source. Next, the pulse height value at one-half of the peak of each spectrum (i.e. the so-called "half-height") is measured and the corresponding pulse height noted. After generation of each pulse spectrum, the count rates for pulses generated from the sample are determined. This is accomplished by counting the sample at various thresholds of pulse height and by extrapolating to a zero threshold in order to obtain the sample count rate at each quench level. Next, the logarithm of the extrapolated count rates are graphically plotted as a function of the relative pulse height for the measured half-heights. A second extrapolation operation is then performed to determine the value of the count rate and hence the disintegration rate at a zero value of relative pulse height.
While it is possible to determine the disintegration rate of a radionuclide in the above manner, the procedure suffers from a number of drawbacks. First, two time consuming extrapolations are required, one to determine the sample count rate at each quench level and another to determine the actual disintegration rate. In addition, measuring the so-called half-height of the Compton spectrum is imprecise and inaccurate. In particular, to measure the half-height, it is first necessary to measure the peak of the Compton spectrum. Such a peak becomes more and more diffuse and difficult to accurately measure as quench increases. Thus, it is necessary to establish a series of adjacent, narrow counting windows to search for the peak and counting must be performed for a long period in each window to obtain statistically valid data for plotting the points defining the peak. Since the half-height is simply the pulse height corresponding to half of the peak height, the half-height is no more accurate than the peak itself.